I am also just about to start my first IVF/ICSI cycle, so I understand where you are coming from.
In regard to 3 days vs. 5 days transfer my understanding is that apparently with 5 day transfers there is a higher success rate, as the embryo is more developed and therefore it is easier to estimate how good the embryo is. BUT there are A LOT OF success stories with 3 day embryos. I think there was a thread on here a little while back about 5 days vs. 3 days and a lot of people seemed to have success with 3 days. If it wasn't successful IVF clinics wouldn't do it. I have heard that sometimes a 3day embryo doesn't develop into a blastocyst, but still could have led to a pregnancy if it had been put back at 3 days. I think it is better to have a 3 day transfer than none at all. So if you have a low number of embryos then it might be better to put one back at 3 days then risk waiting and maybe being left with none.
If you feel comfortable with you FS then I would trust him in what he recommends.